What on earth could have provoked a modern day St. Valentine's Day massacre?

On Valentine's Day, four members of the Coverdale family--George, Jacqueline, Melinda and Giles--were murdered in the space of 15 minutes. Their housekeeper, Eunice Parchman, shot them, one by one, in the blue light of a televised performance of Don Giovanni. When Detective Chief Superintendent William Vetch arrests Miss Parchman two weeks later, he discovers a second tragedy: the key to the Valentine's Day massacre hidden within a private humiliation Eunice Parchman has guarded all her life.